170 ANATOMY FOE, NURSES. [CHAP. XV. 



through the fauces into the pharynx. The elevation of the 

 soft palate prevents the entrance of food into the nasal cham- 

 bers, while the epiglottis bars its entrance into the air passages, 

 and it is guided safely and rapidly through the pharynx into the 

 oesophagus. Here it passes beyond the control of the will ; it 

 is grasped by the oesophageal muscles and by a continuous and 

 rapid peristaltic action is carried onwards and downwards into 

 the stomach. 



Saliva. Mixed saliva (spittle) as it appears in the mouth 

 is a glairy, frothy, cloudy fluid, the glairiness or ropiness being 

 due to mucus ; micro-organisms are also present in it to some 

 extent, and other foreign matters derived from the food. 



Saliva is mainly water containing but little solid matter, its 

 specific gravity varying from 1002 to 1006. It depends for its 

 special action, as a digestive solvent, upon a ferment it contains 

 called ptyalin. 1 



The action of saliva upon the food. The chief function of 

 saliva is to soften and moisten the food and to assist in mastica- 

 tion and deglutition. It has, however, a certain solvent action 

 upon food-stuffs, especially starch. Upon the fats and proteids 

 it has very little effect except to render them softer and better 

 prepared for the action of the other digestive juices. 



By the ptyalin-ferment present in saliva, starch, which is an 

 insoluble substance, is changed into grape sugar, a highly solu- 

 ble and absorbable product. This change is best effected at the 

 temperature of the body, in a slightly alkaline solution, saliva 

 that is distinctly acid hindering or arresting the process. Boiled 

 starch is changed more rapidly and completely than raw, but 

 the food is never retained in the mouth long enough for the 

 saliva to more than begin the transformation of starchy matters. 

 After leaving the mouth, further conversion of starch into sugar 

 is arrested by the acid reaction of the gastric juice, and diges- 

 tion of this class of food-stuffs is practically suspended until 

 they again come in contact with the alkaline secretions in the 

 upper part of the small intestine. 



1 Ferments are either organized or unorganized bodies ; of the former yeast 

 is a well-known example. The fermentative power of yeast which leads to the 

 conversion of sugar into alcohol is dependent upon the life of the yeast-cell. 

 When the yeast-cell dies, fermentation ceases. Ptyalin belongs to the latter 

 class ; it is an inorganic substance, not a living organism like yeast. 



